Video disc systems are known in which a video program, pre-recorded upon a disc, is played back through a television receiver by resort to a procedure which, in general, is similar to the playback of audio records. While known playback systems contemplate a variety of approaches, e.g. optical, capacitive, mechanical (pressure), etc., the present invention finds particular application in the optical system, accordingly, the invention will be described in that environment.
Optical image reproducing systems that permit playback of pre-recorded program material through a television receiver have been proposed to augment the utility of such receivers. The program is stored in a medium such as a disc, not unlike the familiar audio disc, which is interrogated by a beam of coherent light derived from a laser and converged by lens action to a finely focused reading spot. This beam is monitored by a photo detector which serves to develop an electrical signal representative of the stored information. The stored program can include, luminance, chroma and audio information, as well as synchronizing components. This information is positioned in segments of the frequency spectrum which are convenient for disc recording but at the same time subject to transformation in a transcoder so as to assume a frequency distribution typical of a commercial telecast. Since such telecasts feature two interlaced fields for each picture frame, the storage track of the video disc favors the form of a multi-turn spiral in which each convolution of the spiral contains two fields of an image frame together with synchronizing information.
Program information may be stored in the track of an optical video disc in a variety of ways, for example, in formed hill and dale grooves similar to audio recordings or, in a succession of concavities or pits alternating with a like succession of lands disposed along the track. While the invention can be used with either type of recording, there presently appears to be greater interest in the pit and land arrangement, therefore, the invention will be described in connection with a storage track of that format.
The pits and lands constituting the information storage track generally have a uniform width but their length, along the track direction, is variable so that, collectively, the pits and lands comprise a spatial representation of the temporal variations of a pulsed carrier signal modulated in frequency and duty cycle (i.e. density and width of the pulses). This signal conveys the program information and thus is employed to control the formation of the record track on a disc master during the recording process.
The stored information is retrieved by scanning the track with a reading beam and utilizing a photo receptor to respond to beam after it has interrogated the track. In the case where the disc is transmissive to the read beam, the photo receptor can be positioned beneath the disc to collect a portion of the light transmitted there through. On the other hand, where the disc is a reflective device, the photo receptor is located on the same side of the disc as the incident, or reading, beam so as to respond to light reflected from the track. In either case, the pits diffracted (deflect or scatter) the light of the reading beam. In monitoring the degree of scattering or deflection of the light, the photo receptor develops an electrical signal which is modulated in accordance with the stored information.
Although the term scattering is commonly used to describe in general the affect of small pits or irregularities on a beam of incident light, we shall use this term in a more specific sense, that is, as it relates to a particular type of a video disc, the so-called 1/2.lambda. or half wavelength type. In this type of disc the depth of the pit is so chosen that there exists a one-half wavelength optical path difference between rays traversing a pit and rays traversing the adjoining land area. Also, the focused spot of light straddles the track, i.e. it is wider in the radial direction than the pits themselves. It may be shown that under these conditions the incident light is scattered symmetrically in a radial direction (radial scattering) whenever a pit intercepts the focused beam of light. This scattering is minimal when the center of the spot of light coincides with the center of the pit and, as the final information is derived from scattering, we see that this mode of operation detects primarily the center of the pit and determines where the pit edges are by monitoring the decrease in scattered light when the pit moves out from under the beam.
We shall use the term deflection with specific reference to the so-called 1/4.lambda. (one-quarter wavelength) type of disc. In this type of disc, described in U.S. Pat. No. 3,931,459, which issued to the present inventor, the depth of the pit is chosen such that there exists a one-quarter wavelength optical path difference between rays traversing a pit and rays traversing the adjoining land area. The focused beam of light need not straddle the track, at least not for video readout, although it may do so for purposes of deriving information for radial tracking. It may be shown that, in this type of operation, the light is deflected sideways whenever an edge of the pit intercepts the reading beam and, furthermore, that the sense of deflection depends on the position of the land area with respect to the beam. Thus the beginning and end of the pit will deflect the light tangentially in different directions and from this information, detected by a suitable electro-optic system, the radio frequency signal is derived. It will be clear that such a tangential deflection system will detect the edges of the pit rather than the center, as is the case in a radial scattering system. If, in a 1/4.lambda. system, the beam is designed to straddle the track radially, there will also be deflection in the radial direction both toward the inside and the outside of the disc. The relative amount of light deflected in each of these two directions is an indication of the position of the center of the beam relative to the center of the track in a radial sense. Thus, information derived from radial deflection may be used for deriving the error signal needed to keep the beam on track by means of a beam steering servo system.
Although historically the 1/4.lambda. system has been used with transmissive discs and the 1/2.lambda. system with reflective discs, there are no compelling technical reasons for this and in theory transmissive and reflective discs are possible in either system.
An optical video system having particular application to a transmissive type disc is described in U.S. Pat. No. 3,919,562 which issued to Robert L. Whitman on Nov. 11, 1975. On the other hand, an optical video system operating in the reflective mode is described in U.S. Pat. No. 3,959,581 which issued on May 25, 1976 to Leonard J. Laub.
Other reflective mode systems include the arrangements shown in U.S. Pat. Nos. 3,962,720 and 3,969,576. Insofar as these prior are reflective mode systems are concerned, it is readily apparent that they are characterized by rather sophisticated optics which are required in order to separate the reflected diffracted beam from the incident coherent read beam. Specifically, spatial filtering apparatus such as Wollaston prisms or other types of beam splitters are employed to effect the necessary beam separation in prior art reflective mode systems.